Digital subscriber line technology provides the potential for high-speed communication over existing telephone subscriber lines (also referred to as loops or the copper plant). However, time-varying noise conditions can severely impact on DSL performance. Such transient and impulse noise conditions caused, for example by crosstalk from neighbouring lines, the switching on or off of home appliances or even of fluorescent lights cause errors in the data transmission. If these errors are sufficiently severe, they can result in the line having to be re-trained or reset. The transient nature of this noise means that, while it may cause the line to retrain, it is often no longer present at the time the line is reinitialised. As a consequence, the line could be reset to its original data rate leaving it susceptible to further retrains when noise occurs again, and thus inherently unstable. This instability is more apparent at high data rates, as more of the available transmit power is utilized for data transmission rather than for transmission robustness, making the service more sensitive to noise. Yet often those applications requiring a higher bandwidth, such as Voice over IP or IPTV, are severely degraded when a line is unstable. Line instability is thus costly for the operator as instability limits the services an operator can offer successfully over its copper plant. Moreover, as the intermittent noise causing instability is present on some lines, but not all, there may be a need for manual intervention on a case-by-case basis.
Most DSL standards employ discrete multi-tone modulation (DMT), which partitions the channel into a number of parallel sub-channels. Each tone is used to transmit an amount of information that is determined according to the signal-to-noise ratio (SNR) on that sub-channel. The bit rate is determined by the number of bits allocated to each tone (also called the bit-load or bit loading). The bit rate is limited by the transmit power and is inversely dependent on noise power. Thus for any given transmit power, the available bit rate depends on the noise on the line in question.
In order to mitigate the effects of a fluctuating noise level, DSL systems conventionally allocate a target noise margin (in reality a SNR margin) to each DSL line. This margin is essentially the amount of noise increase that a DSL system can tolerate while maintaining a guaranteed bit error rate (BER). This noise margin is applied in the bit loading calculation above the noise level at the time of initialisation. If the noise power increases by a factor that is higher than this allocated margin, the DSL transceiver usually restarts. The target noise margin is conventionally allocated by a transceiver on start-up or following a reset after completing the initialisation procedure. In essence, the target margin defines how much power will not be used for information transmission, but instead serves to protect against noise increases by the same amount. The target margin thus determines the power that is allocated for bit loading, which is also set when initialising the line.
An unstable line can benefit from a higher target margin. Conversely, if the margin value is too high, this limits the bit rate unnecessarily and thus restricts the services that can be offered by the operator. Two of the most commonly used techniques for setting the target noise margin are automatic margin adaptation (AMA) and tiered rate adaptation (TRA). These techniques are examples of level 1 dynamic spectrum management (DSM), which is the optimisation of single lines by means of adjusting control parameters. Both methods share the same basic principle of monitoring an individual line to determine iteratively a set of control parameters (also referred to as a profile) which can provide stability. These methods are described and compared in NICC ND 1513 (2010-01) “Report on dynamic spectrum management (DSM) methods in the UK access network”.
AMA monitors a DSL transceiver for packet errors over a set time interval and sets the target margin as part of a profile according to the number of retrains it suffers. If the line continues to suffer retrains with this profile, the line is again re-initialised with a new profile having a higher target margin. This process continues until the number of packet errors falls within the prescribed thresholds and a further retrain does not occur. In other words, the target margin is increased in a step-wise fashion after each retrain until a value is found that is sufficient to protect this line from any noise condition to come. As the margin is increased, then either power usage increases or data rate decreases. If the DSL is already operating at full power, the data rate will decrease with the increase in margin.
A problem with AMA is that unexpected retrain events can cause the target noise margin value to be increased to very high levels, such as 15 or 18 dB. While the line is protected from most retrains, these high margin values limit the achievable bit rates. If noise conditions improve, i.e. the noise power decreases, this line remains stuck at a low bit rate. If AMA converges, all managed lines are configured with a target margin which is sufficient to cover any noise condition to come. However, these target margins are kept constant and do not take the actual noise condition during showtime, i.e. the real noise level experienced during data transmission, into consideration. This leads to lines using full power, generating more crosstalk in the network and obtaining a suboptimal performance.
TRA works by determining the maximum bit rate that can be supported by a line without retraining. The bit rate is then set to a value that is lower than the determined maximum bit rate. Lines managed with TRA have their target noise margin set to a low value, typically 6 dB. Thus the overall transmit power can be lower than for an equivalent bit rate using AMA. However, any excess power resulting from the bit rate limitation essentially forms part of the SNR margin value, protecting the line against noise variations. In other words, TRA indirectly affects the margin size by defining a safe maximum bit rate.
TRA caps the bit rate value to a level which makes retrains unlikely to occur. By defining bit rate constraints TRA indirectly affects margin values, but eliminates the problem of lines getting stuck at low rates, since the bit rate setting is likely to be supported under all considered circumstances. However, in practice as TRA converges, lines may be limited to the worst-case bit rate. TRA is also sensitive to the amount of power allocated to the line. The higher the transmit power and maximum SNR margin are, the more stable the line will be. However allocated power is not directly controlled by the operator so unexpected retrain events may still occur.